1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and its fabrication method and, more particularly, to an LCD capable having a simplified fabrication process, improved production yield and improved luminance.
2. Description of the Related Art
In the recent information society, a display device as a significant visual information transfer medium has become increasing important, and to take predominance in the market, the display device must meet requirements such as low power consumption, thin profile, light weight and high picture quality. A liquid crystal display (LCD), a major product of a flat panel display (FED), satisfies these conditions and is suitable for mass production as well. As a result, various types of new products of LCDs have been introduced and are replacing the existing cathode ray tubes (CRTs).
In general, the LCD devices display a desired image by controlling light transmittance of liquid crystal cells by separately supplying a data signal according to image information to the liquid crystal cells arranged in a matrix form. An active matrix (AM) method, a driving method typically used for the LCD device, is a method in which liquid crystals of a pixel portion are driven using an amorphous silicon thin film transistor (a-Si TFT) as a switching device. However, the electrical mobility of the amorphous silicon TFT cannot support a peripheral circuit requiring a high speed of 1 MHz or higher. Thus, research is actively being pursued to integrate both the pixel portion and the driving circuit portion on a glass substrate using polycrystalline silicon having a greater field effect mobility than with amorphous silicon. The polycrystalline silicon TFT technique is advantageous in that a driving circuit can be directly fabricated on the glass substrate due to the low photosensitivity and the high yield effect mobility of polycrystalline silicon.
The increase in mobility can enhance an operation frequency of the driving circuit that limits the number of driving pixels, and thus, the image resolution (minuteness or fineness) of the display device can be increased. In addition, because the time for charging a signal voltage of the pixel portion is reduced, distortion of a transmission signal is reduced, and thus, picture quality can be improved.
The structure of an LCD device will now be described with reference to FIG. 1. FIG. 1 is a schematic plan view showing the structure of a related art LCD device, specifically, a driving circuit-integrated LCD device in which a driving circuit portion is integrated on an array substrate.
As shown, the LCD device comprises a color filter substrate 5, an array substrate 10 and a liquid crystal layer (not shown) formed between the color filter substrate 5 and the array substrate 10. The array substrate 10 includes a pixel portion 35, an image display region in which unit pixels are arranged in a matrix form, and a driving circuit portion 30 having a data driving circuit unit 31 and a gate driving circuit unit 32 positioned at the periphery of the pixel portion 35. Though not shown, the pixel portion 35 has a plurality of gate lines and data lines arranged vertically and horizontally to define a plurality of pixel regions on the substrate 10, a TFT as a switching device formed at each crossing of the gate lines and the data lines, and a pixel electrode formed at the pixel region.
The driving circuit portion 30 of the array substrate 10 is protruded as compared with the color substrate 5 and positioned at an outer edge of the pixel portion 35 of the array substrate 10. In this case, the data driving circuit unit 31 is positioned at the longer side of the protruded array substrate 10 and the gate driving circuit unit 32 is positioned at the shorter side of the protruded array substrate 10.
To suitably output an input signal, the data driving circuit unit 31 and the gate driving circuit unit 32 use a TFT of a CMOS (Complementary Metal Oxide Semiconductor) structure, namely, an inverter. A CMOS is an integrated circuit having a MOS structure used for the driving circuit portion TFT which requires a high speed signal processing. The CMOS needs an n-channel TFT and a p-channel TFT, and has speed and density characteristics between an NMOS and a PMOS. The gate driving circuit unit 32 and the data driving circuit unit 31 supply a scan signal and a data signal to the pixel electrode through the gate lines and the data lines. Connected with an external signal input terminal (not shown), the gate driving circuit unit 32 and the data driving circuit unit 31 control an external signal input through the external signal input terminal and output it to the pixel electrode.
A color filter (not shown) for implementing color and a common electrode (not shown), a counter electrode of the pixel electrode formed on the array substrate 10, are formed on the pixel portion 35 of the color filter substrate 5. A cell gap is provided between the color filter substrate 5 and the array substrate 10 to uniformly separate the substrates 5 and 10 using spacers (not shown). The two substrates 5 and 10 are attached by a seal pattern (not shown) formed at an outer edge of the pixel portion 35, thereby forming a liquid crystal display panel. At this time, the attachment of the two substrates 5 and 10 is made by an attachment key formed at the color filter substrate 5 or the array substrate 10.
Since the above described driving circuit integrated LCD device uses a polycrystalline silicon TFT, it has excellent image quality and consumes less power. However, the driving circuit-integrated LCD device is disadvantageous because the n-channel TFT and the p-channel TFT are formed together on the same substrate. Thus, its fabrication process is quite complicated as compared with the amorphous silicon TFT LCD device in which only a single type channel is formed. In addition, a plurality of photolithography processes must be performed to fabricate the array substrate, including the TFTs. The photolithography processes are processes for forming a desired pattern by transferring a pattern drawn on a mask onto the substrate with thin film deposited thereon. The processes included coating a photosensitive solution, exposing photoresist and then developing the photoresist. As a result of the number of delicate steps, production yield is degraded. In particular, the mask designed for forming the pattern is high-priced, so the fabrication cost of the LCD device increases proportionally with the number of masks applied.